1. Field of the Invention
The present invention relates to a conductive resin composition used suitably for electronic apparatus such as an encoder and a contact board that uses the same.
2. Description of the Related Art
Conventional conductive resin composition contains dispersed silver powder in binder resin wherein the silver powder in the conductive resin composition contributes to the conductivity of the conductive resin composition. Silver powder is formed in the shape of flake particle or dendrite particle to thereby increase the contact surface between silver powders and reduce the conduction resistance of the conductive path formed between silver powders.
A contact board to be mounted on an electronic apparatus such as an encoder is fabricated by forming a contact consisting of the abovementioned conductive resin composition on an insulation board, and it is required to cover the surface with carbon overcoat to prevent the silver powder from sulfurization.
However, the conventional conductive resin composition contains flake form or dendrite silver particles, such silver particle having complex configuration is not easily mixed with binder resin. As the result, the silver powder content is limited at most to approximately 30 to 35% in volume, and it is difficult to improve the conductivity further. Furthermore, not only a contact consisting of the conventional conductive resin composition requires carbon overcoat but also the carbon overcoat causes the increased contact resistance of a contact board in contact with a contact brush.
The object of the present invention is to provide sulfurization-resistant conductive resin composition that requires no carbon overcoat and additionally that is excellent in wear resistance, and to provide a contact board formed of the conductive resin composition.
Conductive resin composition contains silver powder that is a mixture of spherical particles and dendrite particles dispersed in binder resin, wherein the content (% by volume) of the spherical particle is larger than the content of the dendrite particle.
The conductive resin composition as described hereinabove is rendered conductive by the silver powder, and the binder resin binds the silver powder. The silver powder of spherical particles forms a conductive path in the conductive resin composition, and furthermore dendrite particles are interposed between spherical particles to thereby increase the electrical contact point between spherical particles, and as the result the conduction resistance is reduced. Furthermore, because the silver powder contains a lot of spherical particles of simpler configuration, the binder resin binds the silver powder easily, and it is possible to improve the sulfurization resistance of the conductive resin composition by increasing the content of the silver powder.
The conductive resin composition of the present invention contains the binder resin and silver powder, and the content of the silver powder ranges from 40 to 50% by volume.
In the conductive resin composition as described hereinabove, because the content of the silver powder is equal to or larger than 40% by volume, the sufficient conductivity is obtained, and because the content of the silver powder is equal to or smaller than 50% by volume, the binder resin can maintains the force to bind the silver powder.
In the conductive resin composition, the content of the silver powder of the dendrite particle ranges from 5 to 18% by volume, and the content ratio of the silver powder of the spherical particle to the silver powder of the dendrite particle ranges from 1.5 to 7.
Because the content of the dendrite particle is equal to or smaller than 18% by volume in the conductive resin composition as described hereinabove, the binder resin binds the silver powder easily. Furthermore, the content ratio of the spherical particle to the dendrite particle ranges from 1.5 to 7 and the content of the dendrite particle is equal to or larger than 5% by volume, sufficient dendrite particles are interposed between spherical particles, and the conduction resistance of the conductive path between spherical particles is reduced.
The conductive resin composition of the present invention contains the silver powder that is a mixture of the spherical particle having an average particle diameter of 3 to 6 xcexcm and the dendrite particle having an average particle diameter equal to or smaller than 1 xcexcm.
In the conductive resin composition as described hereinabove, dendrite particles fills the space between spherical particles sufficiently.
A contact board of the present invention is provided with an insulating board on which a contact comprising any one of the abovementioned conductive resin compositions is formed.
Because such a contact of a contact board is excellent in the sulfurization resistance, it is not necessary to provide a carbon overcoat for preventing the sulfurization and the contact resistance due to contact with a contact brush overcoated by the carbon overcoat is reduced, and a fine contact pattern can be formed. Because the contact is excellent in the conductivity, the conduction resistance between the contact and a contact brush is low, and the reliability of an electronic apparatus that is provided with the contact of the present invention is improved. Furthermore, because the contact is excellent in the wear resistance, a long life electronic apparatus is realized.
The contact of a contact board of the present invention is formed by means of heat pressing technique.
In such a contact board, air holes formed in the contact during a screen printing process are squeezed out to form a dense and hard film with improved wear resistance, and the sulfurization resistance is improved because the space through which sulfurization atmosphere penetrates is eliminated, and furthermore silver powders come closer to each other to result in increased conductivity.
The contact is formed by means of heat pressing technique by applying a heat pressing condition of a heating temperature ranging from 175 to 185xc2x0 C., a pressure ranging from 80 to 200 kg/cm2 and a heat pressing time ranging from 15 seconds to 3 minutes.
The insulating board of such a contact board is prevented from being deformed and thermally deteriorated, and the contact surface is finished smooth. The smooth surface of the contact mounted on an electronic apparatus prevents the noise that occurs when the contact brush rides on the rough surface of the contact, and the reliability of the electronic apparatus is improved.